Re Roenall: you could wait, but it doesn't matter, time limits for the castle/ thieves guild etc aren't counted while you're away from Athkatla, so don't worry that he'll sack your castle while you're away.
Re shops restocking: the adventurer's mart is restocked, but all the old stuff is still there, and no other shops are restocked.
Re shopping in the underdark: yes there are some good things to buy - spells, weapons, etc.
Re stocking up for spellhold: spoiler
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You will get Imoen back shortly after getting there. If you're planning to put her in your group, bring items for her - robe, weapons, etc. Also she'll need spells, so bring scrolls of spells you want her to have.
If you're not getting her back, don't worry about stocking up on arrows etc, you can buy stuff there.
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Goddess of Undress
Co-owner of the Evil Petting Zoo and Sorceress of the HADB
OLD COOTS eye candy and Disrobed Division of the ORT
And her sidekick Pink
"High hopes...an endless river...forever and ever"